Phase retardance optical scanner

ABSTRACT

An optical scanning system includes a radiating source capable of outputting a light beam, a first time varying beam reflector that is configured to reflect the light beam through a scan lens towards a transparent sample at an incident angle that is not more than one degree greater or less than Brewster&#39;s angle of the transparent sample, and a second time varying beam reflector that is configured to reflect the light beam reflected from the transparent sample after passing through a de-scan lens onto a phase retardance detector. The output of the phase retardance detector is usable to determine if a defect is present on the transparent sample. The first time varying beam reflector causes a first phase retardance of the light beam and the second time varying beam reflector causes a second phase retardance of the reflected light beam in the opposite direction of the first phase retardance.

TECHNICAL FIELD

The present invention generally relates to systems and methods for detecting defects in thin transparent materials. More specifically, the present invention relates to detecting defects in thin transparent materials by way of measuring phase change in light reflected from the thin transparent material.

BACKGROUND INFORMATION

Many thin films are used in high technology products. For example, thin films on glass are used in many high technology products such as televisions, monitors, and mobile devices. Inspecting glass is challenging due to its low reflectivity and high transparency. Previous techniques perform glass inspection that requires the glass sample to be spun. Spinning a glass sample introduces problems for glass samples that are fragile, not symmetric, or large. Regardless of these problems, glass samples that are fragile, not symmetric, or large need to be tested for defects before used in costly manufacturing processes and integrated into expensive high technology products.

SUMMARY

In a first novel aspect, an optical scanning system includes a radiating source capable of outputting a light beam, a first time varying beam reflector that is configured to reflect the light beam through a scan lens towards a transparent sample at an incident angle that is not more than one degree greater or less than Brewster's angle of the transparent sample, and a second time varying beam reflector that is configured to reflect the light beam reflected from the transparent sample after passing through a de-scan lens onto a phase retardance detector. The output of the phase retardance detector is usable to determine if a defect is present on the transparent sample. The first time varying beam reflector causes a first phase retardance of the light beam and the second time varying beam reflector causes a second phase retardance of the reflected light beam in the opposite direction of the first phase retardance.

In one example, the optical scanning system further includes a memory circuit and a processor circuit adapted to read information received from the phase retardance detector, and determine if a defect is present on the transparent sample.

In a second novel aspect, an optical scanning system includes a radiating source capable of outputting a light beam, a time varying beam reflector that is configured to reflect the light beam through a scan lens towards a transparent sample at an incident angle that is not more than one degree greater or less than Brewster's angle of the transparent sample, and a focusing lens configured to be irradiated by light scattered from the transparent sample at an angle that is normal to the plane of incidence of the moving irradiated spot on the transparent sample. A first portion of the light beam is scattered from a first surface of the transparent sample and a second portion of the light beam is scattered from a second surface of the transparent sample. A spatial filter is configured to block the second portion of the light beam scattered from the second surface of the transparent sample.

In one example, the optical scanning system further includes a memory circuit and a processor circuit adapted to read information received from the detector and determine if a defect is present on the first surface of the transparent sample.

Further details and embodiments and techniques are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention.

FIG. 1 is a cross-sectional diagram of a thin film deposited on top of a glass sample.

FIG. 2 is a graph illustrating the relationship between phase retardance and the incident angle of irradiating light for a bare glass sample, a 10 Å thin film deposited on a glass sample, and a 100 Å thin film deposited on a glass sample.

FIG. 3 is a diagram of a phase retardance optical inspector.

FIG. 4 is a diagram of a bi-cell detector.

FIG. 5 is a graph illustrating phase retardance caused by both the input mirror and the output mirror when the mirrors are operating in an out-of-phase manner. More specifically, the graph illustrates the relationship between the phase of a beam reflecting off the input and output mirror versus the rotation angle of the input mirror.

FIG. 6 is a graph illustrating the combined phase retardance for both in-phase mirror operation and out-of-phase mirror operation. More specifically, the graph illustrates the relationship between the resulting phase of the beam after being reflected by both the input and output mirrors versus the rotation angle of the input mirror.

FIG. 7 is a graph illustrating the relationship between total phase retardance of the phase retardance optical inspector versus the field of view of the phase retardance optical inspector at the location of the phase retardance detector.

FIG. 8 is a table listing an example of the angles of rotation for both the input mirror and the output mirror when operated in an out-of-phase manner.

FIG. 9 is a diagram of a beam retardance mapping illustrating the detection of defects by way of detecting changes in the beam retardance.

FIG. 10 is a diagram of a scattered radiation optical inspector.

FIG. 11 is a diagram of a scattered radiation mapping illustrating detection of defects by way of detecting changes in the intensity of scattered radiation.

FIG. 12 is a flowchart illustrating the steps to perform phase retardance defect detection.

FIG. 13 is a flowchart illustrating the steps to perform scattered radiation defect detection.

DETAILED DESCRIPTION

Reference will now be made in detail to background examples and some embodiments of the invention, examples of which are illustrated in the accompanying drawings. In the description and claims below, relational terms such as “top”, “down”, “upper”, “lower”, “top”, “bottom”, “left” and “right” may be used to describe relative orientations between different parts of a structure being described, and it is to be understood that the overall structure being described can actually be oriented in any way in three-dimensional space.

Many high technology products involve depositing films on glass or other transparent substrates. An important process control metric is to measure the film thickness and film defects on the glass substrate. This has proven to be difficult due to the low reflectivity of glass and the difficulty of separating signals from the top surface of the glass substrate from signals from the bottom surface of the glass substrate. Another issue with measuring the film thickness and film defects on the glass substrate is that the current techniques do not allow for scanning of many different shapes and sizes of transparent samples.

A solution is needed that: (i) accurately separates signals from the top surface of the glass substrate from signals from the bottom surface of the glass substrate, (ii) detects the presence of defects in response to small changes in signals from the surface of the glass substrate, and (iii) allows for scanning of many different shapes and sizes of transparent samples.

The present invention provides a solution to this problem by providing a scanning method that irradiates the transparent sample at, or near, the Brewster's angle of the transparent sample. This scanning method of irradiating the transparent sample at, or near, Brewster's angle also provides a scan in the x-y coordinate system, which makes the present invention capable of scanning any object shape or object size that is substantially flat.

Transparent sample surfaces, such as glass, frequently have thin films deposited upon their surfaces. FIG. 1 is a cross-sectional diagram of a thin film deposited on top of a glass sample. It is desirable to be able to inspect the transparent sample surface both before deposition for surface cleanliness and after the deposition to check and for film defects. In order to achieve this goal, it is necessary that the technique be very sensitive to films on the transparent sample surface. It is also necessary that that the technique be able to separate the received signal from the top surface of the transparent sample from the received signal from the bottom surface of the transparent sample.

The sensitivity to films on a transparent sample can be addressed by considering the information illustrated in FIG. 2. FIG. 2 is a graph illustrating the relationship between phase retardance and the incident angle of irradiating light for a bare glass sample, a 10 Å thin film deposited on a glass sample, and a 100 Å thin film deposited on a glass sample. More specifically, FIG. 2 shows the retardance (Φ_(P)-Φ_(S)+π) versus the angle of incidence for a typical transparent sample, such as glass (BK7) with a MgF₂ film layer thickness as a parameter. On a glass substrate, the retardance reduces to just the phase of the P wave (Φ_(P)). One conclusion to draw from FIG. 2 is that to detect films on transparent samples, such as glass, it is desirable to operate with a polarization that is near P polarization (polarization that is parallel to the plane of incidence). Another conclusion to draw from FIG. 2 is that it is desirable to perform the scan by irradiating the transparent sample at an incident angle that is at, or near, the Brewster's angle of the transparent sample. For example, ten times more (10×) sensitivity can be achieved by operating at 57 degrees instead of at 60 degrees. Sensitivity is defined as the difference between the 10 Angstrom film curve and the bare glass curve. There is, however, a tradeoff given that operation at exactly Brewster's angle in P polarization will result in no signal being reflected from the transparent solid. Therefore, it is desirable to perform the scan by irradiating the sample at an angle of incidence that is no more than one degree greater or less than Brewster's angle for the transparent sample. It is also desirable to perform the scan by irradiating the sample with a polarization that is no more than 20 degrees from P polarization.

FIG. 3 is a diagram of a phase retardance optical inspector. The phase retardance optical inspector includes a radiating source 10, a half-wave plate 28, a feed-in mirror 11, an input mirror (input time varying beam reflector) 12, a scan lens 13, a de-scan lens 15, an output mirror (output time varying beam reflector) 16, a feed-out mirror 17, a focusing lens 18, a blocker 19 located at a focal plan 20 of the focusing lens 18, a collimating lens 21, a half-wave plate 22, a phase retardance measuring unit 29, a processor 26 (optional), and a memory 27 (optional). The phase retardance measuring unit includes a polarizing beam splitter 23, a first detector 24 and a second detector 25.

In one example, the input 12 and output 16 mirrors are linear time varying beam reflectors, which vary the angle of reflection linearly as they are rotated. Input 12 and output 16 mirrors may also be controlled by an electrical signal, such as a signal generator. Input 12 and output 16 mirrors may be referred to as galvanometer mirrors.

In one example, the first and/or second detectors are bi-cell detectors. An example of a bi-cell detector is illustrated in FIG. 4. A bi-cell detector has a center line 31 that separates a first photo sensor from a second photo sensor. The bi-cell detector can be configured so that it outputs a first signal that indicates the difference between the light intensity measured by one side of the detector and the light intensity measured by another side of the detector. The bi-cell detector also can be configured so that it outputs a second signal that indicates the summation of the light intensity measured by one side of the detector and the light intensity measured by another side of the detector.

In operation, the phase retardance optical inspector measures retardance by measuring the change in polarization of the signal from the transparent sample that results from irradiation of the transparent sample by a scanning beam as it travels across a transparent sample. In order to accurately measure the change in polarization due to the signals from the transparent solid, and not due to the phase retardance caused by the inspector itself, it is required that the optics that produce the moving beam and the optics that de-scan and guide the signals from the transparent solid produce minimal polarization change (retardance).

A major source of polarization change (retardance) caused by the phase retardance optical inspector is the input 12 and output 16 mirrors. The polarization change (retardance) caused by input mirror 12 and output mirror 16 is illustrated in FIG. 5. Input mirror 12 and output mirror 16 can be operated in any desired manner based on their electric control signals, however, the manner in which the input 12 and output 16 mirrors are operated has a large impact on the polarization change (retardance) introduced by the phase retardance optical inspector. The polarization change (retardance) produced by each mirror is a function of the angle of incidence of the light upon the mirror. The larger the angle of incidence, the larger the polarization change (retardance).

In one example, the input mirror 12 and the output mirror 16 could be operated such that the both mirrors operate in-phase, such that each mirror is rotated so that each mirror has the same angle with respect to the beam. This “In-Phase” operation of the input 12 and output 16 mirrors causes maximum polarization change (retardance) because as the input mirror 12 rotates to increase its angle with respect to the beam the amount of polarization change caused by the input mirror increases, and as the output mirror 16 rotates to increase its angle with respect to the beam the amount of polarization change caused by the output mirror increases. Therefore, the polarization change (retardance) caused by each mirror is always in the same direction and results in a maximum polarization change. This maximum variation of phase change (retardance) during “In-Phase” operation is illustrated in FIG. 6. FIG. 6 clearly shows that as the angle of mirror rotation increases, so does the polarization change (retardance).

In another example, the input mirror 12 and the output mirror 16 could be operated such that the mirrors operate out-of-phase, such that each mirror is rotated so that each mirror has the opposite angle with respect to the beam. FIG. 8 is a table listing an example of the angles of rotation for both the input mirror and the output mirror when operated in an out-of-phase manner. This “Out-of-Phase” operation of the input 12 and output 16 mirrors causes minimum polarization change (retardance) because as the input mirror 12 rotates to increase its angle with respect to the beam the amount of polarization change caused by the input mirror increases, and as the output mirror 16 rotates to decrease its angle with respect to the beam the amount of polarization change caused by the output mirror decreases. Therefore, the polarization change (retardance) caused by each mirror is always in the opposite direction and resulting in a minimum. This minimum variation of phase change (retardance) during “Out-of-Phase” operation is also illustrated in FIG. 6. FIG. 6 clearly shows that as the angle of mirror rotation increases, polarization change (retardance) does not exceed two degrees.

With respect to the Out-of-Phase operation, it is also noted that not only is the polarization change (retardance) across the field of view reduced by this technique, but also the reflectivity variation caused by each mirror is reduced as well. This is because the reflectivity of the input mirror is decreasing as its angle of incidence increases and the output mirror reflectivity is increasing since its angle of incidence is decreasing. These two effects will nearly cancel one another resulting in a very minimal change in reflectivity versus angle of incidence.

Another major source of polarization change (retardance) caused by the phase retardance optical inspector is the feed-in angle of the scanning beam upon the input mirror 12. As discussed above, the polarization change (retardance) is reduced as the angle of incidence approaches zero degrees. However, from a practical point of view, feed-in angles of approximately five degrees are possible. In the phase retardance optical inspector the feed-in angle is controlled by the configuration of the radiating source 10 and feed-in mirror 11. In one example, the radiating source 10 and feed-in mirror 11 are configured so that the resulting feed-in angle to input mirror 12 is twelve degrees. A fixed feed-in angle of twelve degrees causes a minimal polarization change (retardance) by the input mirror 12.

In one example, the light reflected by the feed-in mirror irradiates the input mirror 12 at an angle that is not greater than thirty degrees from the normal angle of the input mirror 12 (first time varying beam reflector) when the input mirror 12 is positioned at a mid-point of the input mirror 12 rotational range.

Similarly, yet another major source of polarization change (retardance) caused by the phase retardance optical inspector is the feed-out angle of the signal from the transparent sample upon the output mirror 16. As discussed above, the polarization change (retardance) is reduced as the angle of incidence approaches zero degrees. However, from a practical point of view, feed-out angles of approximately five to fifteen degrees are possible. In the phase retardance optical inspector the feed-out angle is controlled by the configuration of the de-scanning lens 15, output mirror 16, and feed-out mirror 17. In one example, the de-scanning lens 15, output mirror 16, and feed-out mirror 17 are configured so that the resulting feed-out angle to output mirror 17 is twelve degrees. A fixed feed-out angle of twelve degrees causes a minimal polarization change (retardance) by the feed-out mirror 17.

The combination of the Out-of-Phase operation of the input 12 and output 16 mirrors with the minimal feed-in and feed-out angles result in an optical inspector that produces minimal polarization change (retardance). An example of the resulting polarization change (retardance) at the phase retardance measuring unit 29 versus field of view of the optical inspector is illustrated in FIG. 7. This clearly shows the advantage of out-of-phase mirroring versus in-phase mirroring.

The half-wave plate 28 can be used to adjust the polarization of the scanning beam output by the radiating source 10. In one example, the half-wave plate 28 is used to adjust the polarization of the scanning beam to be as close as possible to P polarized. As discussed above, it is advantageous to scan a transparent sample with a near P polarized scan beam.

The scan lens 13 operates to focus the scanning beam onto the transparent sample. In one example, the scan lens 13 is a telecentric scan lens. Scan lens 13 is configurable such that the scanning beam output from the scan lens 13 irradiates the transparent sample at an angle that is not more than one degree from Brewster's angle of the transparent sample. Another type of lens which may replace the scan lens is an achromat.

In one example, the transparent sample is glass. In another example, the transparent sample is a thin film deposited on a transparent material. Other examples of transparent samples include but are not limited to: sapphire, fused silica, quartz, silicon carbide, and polycarbonate.

De-scan lens 15 operates to focus the signal from the transparent sample onto output mirror 16. In one example, the de-scan lens 15 is an achromat. An achromat can be used for de-scanning because critical focusing and telecentricity is not needed when receiving the signal from the transparent sample. This is an economical benefit because an achromat is much less expensive than a telecentric lens. Other examples of a de-scan lens include, but are not limited to: spherical singlet, spherical doublets, triplet, or aspheric lens.

Feed-out mirror 17 operates to reflect the signal from the output mirror 16 to focusing lens 18. Focusing lens 18 has a focal plane 20. At the focus of the focusing lens 18 there will be two spots (provided the sample is transparent) and these spots correspond to signal from the top and bottom surfaces of the sample. In one example, focusing lens 18 is an achromatic lens. Other examples of a focusing lens include but are not limited to: spherical singlet, spherical doublets, triplet, or aspheric lens.

In one example, the light reflected by output mirror 16 irradiates the feed-out mirror at an angle that is not greater than thirty degrees from the normal angle of output mirror 16 (the second time varying beam reflector) when output mirror 16 is positioned at a mid-point of output mirror 16 rotational range.

Blocker 19 is located near the focal plane 20 and operates to block a portion of the signal from the transparent sample that is from a specific surface of the transparent sample. For example, as illustrated in FIG. 3, blocker 19 may be configured so to block signal from the bottom surface of the transparent sample, while allowing the signal from the top surface of the transparent sample to pass. Although not illustrated, the blocker 19 can also be configured so to block signal from the top surface of the transparent sample, while allowing the signal from the bottom surface of the transparent sample to pass. In this fashion, the phase retardance optical inspector is able to differentiate signal from the top of the transparent sample from signal from the bottom of the transparent sample. In one example, the blocker 19 is a mirror. Other examples of a blocker include but are not limited to: an absorbing material, a blackened piece of aluminum, and a black painted piece of metal.

Collimating lens 21 operates to collimate the signal from the transparent sample that is not blocked by blocker 19. In one example, the collimating lens 21 is an achromatic lens. Other examples of a collimating lens include but are not limited to: spherical singlet, spherical doublets, triplet, or aspheric lens.

Half-wave plate 22 operates to adjust the polarization of the signal from the transparent sample before irradiating polarizing beam splitter 23 of the phase retardance measuring unit 29. In one example, the half-wave plate 22 adjusts the polarization of the signal from the transparent sample so that the signals incident upon detectors 24 and 25 are approximately equal.

Upon being irradiated, polarizing beam splitter 23 allows all light polarized in one direction to pass through to detector 25 and reflects all light polarized in the other direction to detector 24. The plane of the polarizing beam splitter 23 is the same as the plane of the sample. Detector 24 outputs a signal indicating the intensity of the light that irradiated detector 24. Detector 25 outputs a signal indicating the intensity of the light that irradiated detector 25. The difference between the signals from the two detectors is proportional to the polarization change (retardance) of the scanning beam caused by defects of the transparent sample or films on the transparent sample. Any small change in the film thickness or properties can be detected by comparing the output signals of detectors 24, 25. The sum of the signals from the two detectors is proportional to the reflectivity of the transparent sample or films on the transparent sample.

In the case where the detectors are bi-cell detectors, the phase retardance measuring unit can also determine a change in the surface slope of the transparent sample.

Processor 26 (optional) can be used to read the output signals from detectors 24 and 25. Processor 26 can execute code that calculates the difference between the output signals and determine if a defect is present on the transparent sample as well as what type of defect the defect is. The processor 26 may also store the intensity values indicated by the output signals in a memory 27 (optional). The processor 26 may also read instructions from memory 27. The processor 26 may also read one or more threshold values to aid in the determination if a defect is present and the type of defect when a defect is present.

FIG. 9 is a diagram of a beam retardance mapping illustrating the detection of defects by way of detecting changes in the beam retardance. This mapping can be created manually based on monitoring the output of the phase retardance measuring unit 29. Alternatively, this mapping can be created automatically by a processor that samples the output of the phase retardance measuring unit 29 and stores the resulting difference computations in memory 27. The beam retardance mapping can then be used to determine if a defect is present on the transparent sample or the thin film deposited on the transparent sample. For example, a decrease in the beam retardance can indicate a change in film thickness, a film defect or a particle defect on the transparent sample. Alternatively, an increase in the beam retardance can indicate a change film thickness, a film defect, or a particle defect on the transparent sample. This beam retardance mapping can also be output as a digital file that is sharable with consumers of the transparent sample.

FIG. 10 is a diagram of a scattered radiation optical inspector. The scattered radiation optical inspector includes a radiating source (not shown) that outputs a source beam 40, a time varying beam reflector 41, a telecentric scan lens 42, a focusing lens 46, a spatial filter 48, a collimating lens 49 and a detector 50.

In operation, the radiating source emits source beam 40 which irradiates time varying beam reflector 41. The time varying beam reflector 41 reflects the source beam 40 to the telecentric scan lens 42. The time variance of the time varying beam reflector 41 causes a moving spot (scanning beam 43) to irradiate transparent sample 44. The time varying beam reflector 41 and the telecentric scan lens 42 are configured so to irradiate the transparent sample 44 with the scanning beam 43 at an angle of incidence that is not more than one degree from the Brewster's angle of the transparent sample 44. The focusing lens 46 is configured to be irradiated by scattered radiation from the transparent sample 44. The scattered radiation is radiated from the top surface of the transparent sample 44, as well as from the bottom surface of the transparent sample 44. The focusing lens 46 can be referred to as a collector of light. In one example, the focusing lens 46 is configured to be oriented along an axis that is perpendicular to the plane incidence of scanning beam 43. In one example, the focusing lens 46 is a low F-number camera lens. The focusing lens 46 focuses light to a focal plane 47. The spatial filter 48 is located at focal plane 47 and operates to filter out the scattered radiation from the bottom surface of the transparent sample 44, while allowing the scattered radiation from the top surface of the transparent sample 44 to pass through to collimating lens 49. The collimating lens 49 is configured along an axis that is perpendicular to the scanning beam 43. In one example, the spatial filter 48 is a slit shaped spatial filter to remove the scattered light from the bottom surface of the transparent sample 44. In another example, the collimating lens 49 is a pair of achromatic lenses that shape the scattered radiation into a circular spot that irradiates detector 50. In yet another example, detector 50 is a photomultiplier tube.

In another example, the scattered radiation optical inspector further includes a processor and a memory. The processor functions to read the output signals generated by the detector 50 and store the light intensity values indicated by the output signals in the memory. The processor may also function to determine the presence of defects and the type of defects. The processor may also function to generate a mapping of defects across the area of the transparent sample. The processor may also be configured to communicate the mapping of defects to another device or to a monitor.

The scattered radiation optical inspector described above gathers scattered radiation from the irradiated transparent sample 44 at an angle that is near perpendicular from the angle of incidence of the scanning beam 43. Moreover, the scattered radiation optical inspector can separate scattered radiation from the top surface of the transparent sample from scattered radiation from the bottom surface of the transparent sample, which provides the valuable ability to detect defects on a single side of a transparent sample.

The scattered radiation optical inspector can be integrated with the phase retardance optical inspector of FIG. 3 because both inspectors require that the transparent sample be irradiated at an angle of incidence that is not more than one degree from the Brewster's angle of the transparent sample.

FIG. 11 is a diagram of a scattered radiation mapping illustrating detection of defects by way of detecting changes in the intensity of scattered radiation from one surface of the transparent sample. For example, the scattered radiation mapping may show a location where there is a decrease in measured intensity by detector 50. This decrease in measured intensity can be an indicator of a change in film thickness, a film defect, or a particle defect. In another example, the scattered radiation mapping may show a location where there is an increase in measured intensity by detector 50. This increase in measured intensity can be an indicator of a change in film thickness, a film defect, or a particle defect.

FIG. 12 is a flowchart 100 illustrating the steps to perform phase retardance defect detection. In step 101, a transparent sample is irradiated at an angle of incidence that is not more than one degree greater or less than the Brewster's angle of the transparent sample. In step 102, an input mirror and an output mirror are rotated so that the input mirror is at a minimum angle of incidence when the output mirror is at a maximum angle of incidence. In step 103, light reflected from the bottom surface of the sample is blocked. In step 104, the retardance (polarization change) of the light reflected from the top surface is measured. In step 105, it is determined if a defect is present at the scan location based on the measured retardance and one or more threshold values.

FIG. 13 is a flowchart 200 illustrating the steps to perform scattered radiation defect detection. In step 201, a transparent sample is irradiated at an angle of incidence that is not more than one degree greater or less than the Brewster's angle of the transparent sample. In step 202, the scattered radiation from the transparent sample is focused using a lens orientated at a normal angle to the plane of incidence of the moving irradiated spot on the sample. In step 203, the scattered radiation from the bottom surface of the transparent sample is blocked. In step 204, the scattered radiation from the top surface of the transparent sample is collimated. In step 205, the intensity of the scattered radiation from the top surface of the transparent sample is measured. In step 206, a determination is made as to whether a defect is present at the scan location based on the measured intensity and one or more threshold values.

Although certain specific embodiments are described above for instructional purposes, the teachings of this patent document have general applicability and are not limited to the specific embodiments described above. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims. 

What is claimed is:
 1. An optical scanning system, comprising: a radiating source capable of outputting a light beam; a first time varying beam reflector that is configured to reflect the light beam through a scan lens towards a transparent sample at an incident angle that is not more than one degree greater or less than Brewster's angle of the transparent sample; and a second time varying beam reflector that is configured to reflect the light beam reflected from the transparent sample after passing through a de-scan lens onto a phase retardance detector, wherein the output of the phase retardance detector is usable to determine if a defect is present on the transparent sample.
 2. The optical scanning system of claim 1, further comprising: a memory circuit; and a processor circuit adapted to: read information received from the phase retardance detector; and determine if a defect is present on the transparent sample.
 3. The optical scanning system of claim 1, wherein the first time varying beam reflector causes a first phase retardance of the light beam, and wherein the second time varying beam reflector causes a second phase retardance of the reflected light beam, wherein the first phase retardance is in the opposite direction of the second phase retardance, and wherein a total phase retardance caused by the combination of the first and second time varying beam reflectors varies by no more than two degrees over a field of view of the optical scanning system.
 4. The optical scanning system of claim 1, wherein the first time varying beam reflector and the second time varying beam reflector are controlled by a function generator.
 5. The optical scanning system of claim 1, wherein the first time varying beam reflector is configured to rotate about an axis that is perpendicular to the incident angle.
 6. The optical scanning system of claim 1, wherein the light beam output by the radiating source is configured to be reflected by a feed-in mirror before irradiating the first time varying beam reflector, wherein the light beam irradiates the first time varying beam reflector at an angle that is not greater than thirty degrees from the surface normal of the first time varying beam reflector when the first time varying beam reflector is positioned at a mid-point of the first time varying beam reflector rotational range.
 7. The optical scanning system of claim 1, further comprising a blocker, wherein a first portion of the light beam reflects from a first surface of the transparent sample, wherein a second portion of the light beam reflects from a second surface of the transparent sample, and wherein the blocker is configured to block the light reflected from the second surface of the transparent sample.
 8. The optical scanning system of claim 1, further comprising: a focusing lens that is configured to focus the light beam reflected from the transparent sample after passing through the de-scan lens and reflecting from the second time varying beam reflector; a blocker located at a focal plane of the focusing lens, wherein a first portion of the light beam reflects from a first surface of the transparent sample, wherein a second portion of the light beam reflects from a second surface of the transparent sample, and wherein the blocker is configured to block the light reflected from the second surface of the transparent sample.
 9. The optical scanning system of claim 8, further comprising a collimating lens configured to be irradiated by the light reflected from the first surface of the transparent solid; and a wave plate configured to be irradiated by the light reflected from the first surface of the transparent solid.
 10. The optical scanning system of claim 7, further comprising: a wave plate configured to be irradiated by the light reflected from the first surface of the transparent sample; a beam splitter configured to be irradiated by the light reflected from the first surface of the transparent sample after passing through the wave plate; a first detector configured to be irradiated by a first portion of the light reflected from the first surface of the transparent sample; and a second detector configured to be irradiated by a second portion of the light reflected from the first surface of the transparent sample, wherein the beam splitter, first detector and second detector are part of the phase retardance detector.
 11. The optical scanning system of claim 10, wherein the wave plate is a half wave plate.
 12. The optical scanning system of claim 10, wherein the beam splitter is a polarizing beam splitter configured to: (i) output the first portion of the light reflected from the first surface of the transparent sample in a first direction toward the first detector, and (ii) output the second portion of the light reflected from the first surface of the transparent sample in a second direction toward the second detector.
 13. The optical scanning system of claim 10, wherein the first detector is a bi-cell detector, wherein the second detector is a bi-cell detector, and wherein the first and second bi-cell detectors are configured to output a total light intensity signal and a difference intensity signal.
 14. The optical scanning system of claim 1, wherein the first time varying beam reflector reflects the light beam linearly across all angles of rotation, and wherein the second time varying beam reflector reflects the light beam linearly across all angles of rotation.
 15. The optical scanning system of claim 1, wherein the first time varying beam reflector is a first flat mirror, and wherein the second time varying beam reflector is a second flat mirror.
 16. The optical scanning system of claim 1, wherein the radiating source is a laser, wherein the scan lens is a telecentric lens, and wherein the de-scan lens is an achromatic lens.
 17. The optical scanning system of claim 9, wherein the focusing lens is an achromatic lens, and wherein the collimating lens is an achromatic lens.
 18. The optical scanning system of claim 1, wherein the light reflected from the transparent sample is configured to be reflected by a feed-out mirror after irradiating the second time varying beam reflector, wherein the reflected light irradiates the feed-out mirror at an angle that is not greater than thirty degrees from the normal angle of the second time varying beam reflector when the second time varying beam reflector is positioned at a mid-point of the second time varying beam reflector rotational range.
 19. An optical scanning system, comprising: a radiating source capable of outputting a light beam; a scan lens positioned to be irradiated by the light beam; a de-scan lens positioned to be irradiated by a portion of the light beam reflected from a transparent sample; a first means for minimizing phase shift of the light beam before irradiating the scan lens; and a second means for scanning the light beam onto the transparent sample and de-scanning light reflected from the transparent sample without causing more than two degrees of phase retardance.
 20. The optical scanning system of claim 19, wherein the first means comprises a feed-in mirror and wherein the second means comprises a first time varying beam reflector and a second time varying beam reflector, wherein the first time varying beam reflector is configured to be at a minimum angle of incidence when the second time varying beam reflector is at maximum angle of incidence. 